Many thanks!
I knew to make sure that the file is executable, but I $PATH was new to me. I tried it now with many different compinations (my own script, the run_linx_program from the wiki link above).
I tried to leave the field empty in SketchUp's preferences and set jpg files to be opened with the run_linx_program script. I get always the error message
"image editor could not be launched" (roughly translated)
Only two possibilities worked:
When I add simply winebrowser "%1" to the registry, I get the error message but the image opens in eye-of-gnome.
When I add winebrowser in SketchUp's preferences, it opens eye-of-gnome and there is no error message.
It seems that the error message appears only when I pass the image to a script, and the only possibility that I found is winebrowser.
I would like to keep an image viewer default for jpg files on my linux desktop. Is it possible to make winebrowser open jpg files in gimp, or to make gimp only default for jpg within wine?